Enhanced proteolysis of IkappaBalpha and IkappaBbeta proteins in astrocytes by Moloney murine leukemia virus (MoMuLV)-ts1 infection: a potential mechanism of NF-kappaB activation.

Moloney murine leukemia virus (MoMuLV)-ts1-mediated neuronal degeneration in mice is likely due to loss of glial support and release of inflammatory cytokines and neurotoxins from surrounding ts1-infected glial cells including astrocytes. NF-kappaB is a transcription factor that participates in the transcriptional activation of a variety of immune and inflammatory genes. We investigated whether ts1 activates NF-kappaB in astrocytes and examined the mechanism(s) responsible for the activation of NF-kappaB by ts1 infection in vitro. Here we present evidence that ts1 infection of astrocytes in vitro activates NF-kappaB by enhanced proteolysis of the NF-kappaB inhibitors, IkappaBalpha and IkappaBbeta. In in vitro studies using protease inhibitors, IkappaBalpha proteolysis in ts1-infected astrocytes was significantly blocked by a specific calpain inhibitor calpeptin but not by MG-132, a specific proteasome inhibitor, whereas rapid IkappaBbeta proteolysis was blocked by MG-132. Furthermore, treatment with MG-132 increased levels of multiubiquitinated IkappaBbeta protein in ts1-infected astrocytes. These results indicate that the calpain proteolysis is a major mechanism of IkappaBalpha proteolysis in ts1-infected astrocytes. Additionally, ts1 infection of astrocytes in vitro increased expression of inducible nitric oxide synthase (iNOS), a NF-kappaB-dependent gene product. Our results suggest that NF-kappaB activation in ts1-infected astrocytes is mediated by enhanced proteolysis of IkappaBalpha and IkappaBbeta through two different proteolytic pathways, the calpain and ubiquitin-proteasome pathways, resulting in increased expression of iNOS, a NF-kappaB-dependent gene.